Long-Term Evolution (LTE) is a wireless communication technology developed by the 3rd Generation Partnership Project (3GPP). LTE defines Time Division Duplex (TDD) and Frequency Division Duplex (FDD) modes of operation. LTE also defines two modes of FDD operation: full duplex FDD and Half Duplex FDD (HD-FDD). Communication between a wireless base station 10 and a user equipment (UE) 20 comprises downlink (DL) transmissions from the base station to the UE and uplink (UL) transmissions from the UE to the base station.
FIG. 2 schematically shows Full duplex FDD and half duplex FDD. In each case there are separate spectral resources for DL and UL communication. In full duplex FDD mode a user equipment (UE) can simultaneously transmit and receive using the separate DL and UL resources. In half-duplex FDD mode, the UE cannot receive and transmit simultaneously at a given time. The time axis is divided into frames and subframes. During each subframe the UE may receive on the DL or transmit on the UL.
HD-FDD is suited to low-complexity UEs. Because in HD-FDD mode a UE is not expected to transmit and receive simultaneously, the UE may only be capable of tuning to one frequency band at a time. For example, a transceiver of the UE may only have a single local-oscillator (LO). Re-tuning the LO between DL spectrum and UL spectrum takes some time, and during this re-tuning period the transceiver is not available for DL reception or UL transmission. A half duplex FDD scheme may use a guard period to allow a UE to switch retune between a downlink and an uplink. In LTE the terminology for HD-FDD implemented with a single LO is known as HD-FDD type B. In LTE HD-FDD Type B a re-tuning gap of one subframe is provided. This is called a guard subframe. FIG. 2 shows a guard subframe for a DL-UL switch and a guard subframe for an UL-DL switch.
There are some further constraints on the achievable downlink data rate in half duplex operation. One constraint is that the UE needs to switch to transmit on the uplink to confirm whether or not it correctly received data on the downlink. In LTE this mechanism is a Hybrid Automatic Repeat Request (HARQ) mechanism. The UE transmits a single HARQ-ACK per downlink transmission. The HARQ-ACK indicates if the downlink transmission was correctly received (ACK) or if the downlink transmission was incorrectly received (NACK). There is a fixed timing relationship between the downlink transmission and the HARQ-ACK reply from the UE.
Another constraint is that a UE receives a control message which schedules a downlink data transmission before receiving the downlink data transmission. Both the control message and the downlink data transmission occur within a group of downlink subframes before the UE switches to an uplink. This can leave unused periods within a group of downlink subframes.
The examples described below are not limited to implementations which solve any or all of the disadvantages of known systems.